Fast-closing valve system for cyclotrons

ABSTRACT

A fast-closing valve system has been provided which operates to protect a cyclotron from loss of vacuum and backwash of radioactive contamination when using intensely alpha-active actinide targets. The valve portion consists of a valve chamber adapted for mounting in a beam tube and having aligned ports on opposite sides through which the accelerated particle beam travels. A valve gate is slidably mounted in the valve chamber between the ports and normally urged into a seated position, closing the ports, by means of compression springs. The valve gate is held open by means of an electromagnet and, when the current to the electromagnet is suddenly shorted, the magnetic field rapidly collapses and the springs slam the valve gate into the seated position. The circuit controlling the actuation of the electromagnet uses a spark plug as a pressure sensor mounted in the beam tube, downstream of the valve and when a slight pressure rise in the beam tube occurs the spark plug fires triggering the control circuit which, in turn, shorts the electromagnet. A plurality of annular baffles are mounted in the beam tube to retard the shock wave from pressurized gas backwash in the case of an accident, such as a target rupture, allowing the valve to close before the gas reaches the valve assembly.

Unites States Hahn et a1.

atent July 4, 1972 R. Tarrant; Lee D. Hunt, all of Oak Ridge, Tenn.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission [22] Filed: Jan.28,197l

21 Appl.No.: 110,690

[52] U.S.Cl ..315/l11,137/487.5, 141/65,

176/11, 250/413, 313/62, 313/63, 324/33, 328/234 [51] lnt.Cl. ..F16k31/06,l-105h 7/10 [58] Field ofSearch ..315/108, 110, 111; 313/7, 61-63,

313/93; 324/33; 328/233-238; 137/4875, DIG. 8; 251/131; 176/11; 250/413, 84, 104; 141/65; 73/389; 174/8, 11

CYCLOTRON Primary Examiner-Roy Lake Assistant Examiner-Siegfried H. Grimm Attorney-Roland A. Anderson [57] ABSTRACT A fast-closing valve system has been provided which operates to protect a cyclotron from loss of vacuum and backwash of radioactive contamination when using intensely alpha-active actinide targets. The valve portion consists of a valve chamber adapted for mounting in a beam tube and having aligned ports on opposite sides through which the accelerated particle beam travels. A valve gate is slidably mounted in the valve chamber between the ports and normally urged into a seated position, closing the ports, by means of compression springs. The valve gate is held open by means of an electromagnet and, when the current to the electromagnet is suddenly shorted, the magnetic field rapidly collapses and the springs slam the valve gate into the seated position. The circuit controlling the actuation of the electromagnet uses a spark plug as a pressure sensor mounted in the beam tube, downstream of the valve and when a slight pressure rise in the beam tube occurs the spark plug fires triggering the control circuit which, in turn, shorts the electromagnet. A plurality of annular baffles are mounted in the beam tube to retard the shock wave from pressurized gas backwash in the case of an accident, such as a target rupture, allowing the valve to close before the gas reaches the valve assembly.

4 Claims, 3 Drawing Figures ELECTRONIC VALVE CONTROL UNIT PATENTEDJUL 41972 SHEET 10F 3 MQ-W 5 n 4! mm mH wm V H D N f a wwmm RLR J ATTORNEY.

PATENTEDJUL 4 I972 SHEET 2 UF' 3 INVENTORS. Richard L. Hahn Lee D. Hunt Roland L.Sione B James R. Tarranf Y ATTORNEY.

F AST-CLOSING VALVE SYSTEM FOR CYCLOTRONS BACKGROUND OF THE INVENTION The present invention was made during the course of, or under, a contract with the United States Atomic Energy Commission.

In research projects with transuranium nuclides, researchers often irradiate targets of alpha-particle emitting isotopes, such as "U, Pa, 'Am, Es and cm. The target material is usually deposited upon a thin foil, which serves as the window between the cyclotron vacuum (=--l Torr) and a chamber containing helium gas at pressures form =0.2 to 2 atm. Thus, the danger exists, should the foil rupture during irradiation, that a shock wave traveling at approximately the sonic velocity in helium would carry the gas and some of the alpha-active material into the cyclotron, causing a shutdown of the complete cyclotron facility. The decontamination of a cyclotron is very time consuming and expensive not only for cost involved in decontamination but loss of operating as well.

Such an occurrence could be prevented by a fast-acting valve placed between the radioactive target and the cyclotron; in the case of target rupture, the valve would close before the shock wave could travel past it. The beam tube and associated vacuum pumps from target to valve would have to be decontaminated which is a relatively minor job, but the cyclotron itself could continue in operation.

Valves that would close an opening in a beam tube inf 6 msec. have been previously constructed; but since their rapid operation results from the ignition of an explosive charge such as gun powder, these valves have to be frequently dismantled and cleaned to remove the residues of the explosive mixture. Also, some of the valve parts tend to deform after a few firings because of the great impact delivered by the moving valve when it slams shut.

SUMMARY OF THE INVENTION It is an object of this invention to provide a fast-closing valve system for protection of a vacuum system which overcomes the disadvantages of the prior art valves.

It is another object of this invention to provide a fast-closing valve system for protection of the evacuated portion of a cyclotron which allows the beam from the cyclotron to pass when open and is triggered closed by a minute rise in the downstream pressure.

Another object of this invention is to provide a fast-closing valve system wherein the valve gate is activated by an electromagnet which is arranged with respect to the valve gate so that upon activation the gate is open in opposition to spring tension means and when the gate is triggered closed, shorting the electromagnet, the valve gate slams shut.

A further object of this invention is to provide a fast-closing valve system for protection of the evacuated area of a cyclotron wherein an electromagnetic operated valve gate is controlled by means of a spark plug sensor positioned downstream of the valve gate for sensing a rise in pressure which causes the spark plug to arc, thereby shorting the electromagnet.

Still another object of this invention is to provide a fast-closing valve system for protection of the internal evacuated area of a cyclotron from the backwash of a gas, possibly containing radioactive material, due to a downstream rupture in the beam tube of the cyclotron wherein a plurality of baffles are disposed downstream of the valve to retard the pressure shock wave from reaching the valve gate before the valve is closed upon sensing a rise in downstream pressure.

Briefly, the present invention is a fast-closing valve system for protecting a vacuum system from the backwash of a gas in case of a sudden rupture in a line of said system, comprising: a valve housing positioned in said line, said housing having therein a valve chamber, aligned ports on opposite sides of an opening into said chamber; a valve gate slidably mounted in said valve chamber; means for normally urging said valve gate into an extended position, closing said opening; means including an electromagnet for holding said valve in a retracted position against the force of said urging means during normal operation, when said electromagnet is energized; pressure sensing means disposed downstream of said housing from said vacuum system and actuated by a minute rise in pressure in said line indicating a rupture for deenergizing said electromagnet, thereby releasing said valve gate; and a plurality of baffles disposed in said line between said valve housing and said pressure sensitive means for attenuating a pressure shock wave generated by a sudden rupture in said line, thereby allowing time for said valve gate to close before said backwash of gas reaches said valve housing.

Other objects and many of the attendant advantages of the present invention will be obvious from the detailed description of a preferred embodiment of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic diagram of a fast-closing valve system according to the present invention;

FIG. 2 is a pictorial view, partially in section, of the electromagnetically operated valve shown schematically in FIG. 1; and

FIG. 3 is a schematic circuit diagram of the electronic valve control unit shown in block from in FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT For purposes of illustrating the preferred embodiment of the invention, the invention will be described as a fast-closing valve system for interrupting the beam in a cyclotron beam tube and providing a gas-tight seal in the event of rupture of a target material positioned in the beam tube which normally provides a seal between the evacuated portion of the cyclotron and a pressurized gas chamber. It will be obvious to those skilled in the art that various other applications of the valve system will be apparent from the detailed description set forth hereinbelow.

Referring now to FIG. 1, the valve system is embodied in a section of a cyclotron beam tube 5 which forms a part of the evacuated system of a cyclotron (not shown) from which a beam of accelerated particles emerge. The beam is directed through a central opening 7 of a fast-closing valve assembly 9, which will be described in detail with reference to FIG. 2 hereinbelow. The valve consists of a valve gate 11 which is slidable disposed in a valve chamber 13 and normally urged into the extended position to close the beam passage opening 7 by means such as a spring 15 (shown in the compressed position). The valve gate 11 is held in the retracted position by means of an electromagnet 17 which holds a connecting rod 19 connected to the valve gate 11, thereby allowing the beam to pass through the opening 7.

The beam is directed through the beam tube 5 until it strikes a target 21 which may be a thin metal foil window which serves as a gas-tight seal between the cyclotron vacuum 10 4 Torr) and a chamber 23 containing helium gas at pressures in the range of approximately 0.2 to 2 atm. The window-foil is usually a Be or Ni foil having the particular radioactive target material deposited thereon. Should the thin window-foil rupture during irridation, the danger exists that a shock wave traveling at approximately sonic velocity (970 m/sec. in helium at N.T.P.) would carry the helium gas and some of the radioactive material into the cyclotron vacuum system.

A pressure sensitive detector, such as a spark gap or a spark plug 25 as used in internal combustion engines, is located just upstream of the target 21 well out of the path of the cyclotron beam so as not to disturb the beam travel or cause the spark gap to fire erroneously.

In actual operation, the valve must close only if the target 21 has been broken; otherwise the beam should not be interrupted. Therefore, the spark plug is threaded through the wall of beam tube 5 as close to the target area as possible and a voltage is impressed across the gap from an electronic control unit 27 which will be described in detail hereinbelow with reference to FIG. 3. Since the pressure in the evacuated area adjacent the target will rapidly rise when the target ruptures, the spark plug will fire signaling the control unit to release the valve gate 11 by shorting the electromagnet 17. It was found that a conventional automobile spark plug, with an electrode gap of 0.01 cm. and an impressed potential of ZOOOV, would fire from helium or air pressures 200 pm of Hg to 1 atrn. Therefore, the spark plug serves as a reliable indicator of the target foils condition.

Timing tests of the system indicate that the valve closesf30 msec. after the target foil is broken. Thus, for a practical distance of approximately 10 meters between the target foil 21 and the valve assembly 9 a closing time of 30 milliseconds is satisfactory for shock waves in air (sonic velocity U 330 m/sec. at N.T.P.). However, in helium the speed is 970 m/sec. Nevertheless, one can still use the valve system for helium by attenuating the shock wave. Experiments in over-pressurized systems have shown that annular baffles are extremely effective in reducing the velocity and pressure of a shock wave. Thus, a plurality of annular baffles 29 are placed in the beam tube near the target chamber 23 and near the valve so that the beam passes freely through the aligned central openings. The important parameter to consider in the use of baffles is the reduction in cross sectional area caused by the introduction of the baffles, i.e., the ratio of the area of the baffle opening 31 to that of the beam tube 5. Also, it was found that several baffles placed at distances of 1 to 3 pipe diameters from each other act in concert and are more effective than when spaced far apart.

Using bafiles in a beam tube may cause some problems, however. The baffles should impede the progress of a shock wave through the tube; they must not prevent the cyclotron beam from reaching the target. Calculations of the beam characteristics at selected distances along a beam tube, including the effects of steering and focussing magnets, were performed with the aid of a computer program. The baffles were then constructed with concentric holes that were twice as large as the calculated maximum beam dimensions to compensate for effects such as the non-reproducibility of the ionsource image and the momentum spread ofthe beam. The baffles, with holes varying from 2.5 to 5.0 cm., were placed near the valve and near the target chamber at intervals of 25.4 cm. in a beam tube approximately meters long and l0.2 cm. diameter.

Referring now to FIG. 2, the preferred embodiment of the valve assembly 9 is shown in detail. The valve housing consists of first and second plates 41 and 43 forming a portion of opposite sides of a valve chamber 45 which is formed on the remaining sides by a central plate 47. The outer plates 41 and 43 are adapted by means of threaded holes 49 to be mounted between a pair of flanges (not shown) of beam tube 5. Each of the plates 41 and 43 has aligned central openings 51 through which the beam passes when the valve is open.

A valve gate 55 is slidably disposed in the chamber 45. It will be noted that valve gate 55 is tapered on opposite sides toward the lower arcuate portion thereof which conforms with the bight of the lower portion of chamber 45 to form a valve seat. The inner portions of plates 41 and 43 are also tapered to conform to the taper of valve gate 55 and provided with O- ring seals 57 so that when the valve gate slams it is wedged against the O-ring seals to prevent the leakage of gas past the valve gate. Once the gate closes, in the position shown, the seal is further aided by the cyclotron vacuum to pull the gate against the O-ring seal on the cyclotron side of the valve. Plates 41 and 43 are further provided with cooling channels 53 through which a portion of the system cooling liquid is passed so that when the valve gate closes, interrupting the beam, the valve gate surface being bombarded by the high energy particles from the cyclotron is cooled.

The outer plates 41, 43 and central plate 47 may be assembled by conventional means, as by bolting the plates together with appropriate gas-tight seals encircling the valve chamber 45 so as to maintain the system vacuum, which the chamber is in communication with, during normal operation, and to prevent the outleakage of gas when the valve is closed.

The valve gate 55 is actuated by means of an electromagnet 57 which is held in place by means of an upper support plate 59. A threaded stud 61, through which the power leads 63 to the coil of the magnet enter, extends through an opening in plate 59 and is secured by means of nut 65. The support plate is held in place by means of mounting rods 67 which extend below the electromagnet 57 and secured at the lower end to the central plate 47. A ferromagnetic drive plate 69 is slidably disposed between the mounting rods 67 under the electromagnet 57 and has a connecting rod 71 which extends downward through an opening 73 into the valve chamber 45 and is secured to the upper end of valve gate 55. An O-ring 75 is provided in a channel of opening 73 to provide a gas-tight seal about rod 71.

The drive plate 69 is provided with bearing sleeves 77 which are slidably disposed about rods 67 and provides free upward and downward movement of the drive plate 69. The drive plate 69 is normally urged downward, holding the valve gate 55 in the extended or closed position, by means of springs 79 coaxially mounted about rods 67 which bear against flanges 81 of sleeves 77 at the lower end thereof and against adjustable collars 83 at the upper end. Collars 83 are adjustably secured to rods 67 in a conventional manner such as by set screws to provide selected loading of springs 79 so that when the electromagnet releases the drive plate 69 it is biased to slam the valve gate 55 into the extended or closed position. A mechanical arrangement, including a pivotally mounted lever having a hook 82 at one end which engages a lifting stud 84 on drive plate 69, is provided to lift the drive plate 69 against the force of bias springs 79 into the retracted position where it is held by electromagnet 57. By mechanically lifting the drive plate 69, the current to electromagnet 69 is held at a minimum value necessary only to hold the valve open, thereby providing faster closing of the valve when the electromagnet is shorted, releasing the valve drive plate 69.

Referring now to FIG. 3, the electronic valve control unit 27 will be described in detail. Basically, the circuit consists of two power supplies, one which supplies the power to the coil 91 of the electromagnet and another which supplies the high voltage to the spark plug 25. The first supply includes a step-down transformer 93 which is connected to a 1 17V A.C. supply through a variable voltage means, such as a variac 95, whereby operating current to coil 91 may be adjusted. The output of transformer 93 is connected across the input of a rectifier bridge 97. One output of bridge 97 is connected to ground potential and the other output is connected through an R-C filter circuit 99 to one terminal of an ammeter 101. The other terminal of ammeter 101 is connected through a pair of series connected resistors 103 and 105 to the ungrounded terminal of coil 91.

The high voltage supply consists of a step-up transformer 107 connected between the 1 l7V supply at the input of a voltage doubling circuit. The voltage doubling circuit consists of a pair of series connected high voltage capacitors 109 and 111 having their common connection point connected to one output terminal of transformer 107. The other terminal of capacitor 109 is connected to the anode of a blocking diode 113 which has its cathode connected to the other output terminal of transformer 107 through a resistor 1 15. The other terminal of capacitor 111 is connected to the cathode of a second blocking diode 117 whose anode is connected to the cathode of diode 113. Thus, the voltage across each of the capacitors will build-up to a voltage near that of the output of transformer 107 (typically 1,000 volts) and the output voltage of the voltage doubling circuit, taken across capacitors 109 and 111, will be approximately twice that of the output of transformer 107 (typically ==2,000V). One output terminal of the voltage doubler is connected to the ungrounded electrode of spark plug 25, or other suitable spark gap device, through series connected current limiting resistors 119 and 121. The other output terminal is connected through a current limiting resistor 123 and a potentiometer 125 to the grounded electrode of spark plug 25. The leads 122 and 124 connecting resistor 121 and potentiometer 125, respectively, to spark plug 25 are shielded cables with the shields connected to ground potential. A voltage regulating zener diode 127 is connected between the common terminals of resistors 119421 and the common terminals of resistor 123 and potentiometer 125, so as to regulate the voltage applied to spark plug 25. Typically, diode 127 may be a series of zener diodes sufficient in number to provide a total zener voltage drop thereacross which equals the desired regulating voltage level.

In order to short the coil 91 and thereby release the valve gate, the adjustable arm of potentiometer 125 is connected to the gate electrode of a silicon controlled rectifier (SCR) 129 through a resistor 131. The anode of SCR 129 is connected to the common connection point of resistors 103 and 105 of the valve coil supply circuit through a normally closed push button reset switch 133 and the cathode of SCR 129 is connected to ground potential. Potentiometer 125 is shunted by a zener diode 135 to limit the voltage drop across the potentiometer.

In operation, when the spark plug 25 fires, capacitors 109 and 111 will discharge through the spark plug circuit, causing a current to flow through potentiometer 125. The arm of potentiometer 125 is adjusted to provide a selected gate voltage for SCR 129 so that is it is triggered On." Once the SCR is On," it will short the valve coil 91 and will continue to conduct until reset by depressing reset switch 133.

In order to test the reliability of the valve system, the target chamber 23 was adapted to be filled with a gas to pressures from a few cm. of Hg to several atmospheres. A thin circular foil, 0.7 cm, in diameter, served as a surrogate target, separating the gas-filled target chamber from the evacuated beam tube 5. To induce a shock wave traveling down the tube, a sharp edged plunger was used to break the foil suddenly. The valve assembly 9 was separated from the target chamber 23 by 10.4 meters of 10.2 cm. diameter beam tubing. When the foil was broken, the sudden increase in pressure adjacent the foil window caused the spark plug to fire and release the valve gate 11 (FIG. 1). In order to verify that there was no gas escaping past the valve assembly, a helium leak detector was connected to the system upstream of the valve. The target chamber was then filled with the helium at various pressures from approximately 0.5 to 2 atrn. and following each filling the target was ruptured, no indication of any helium passing the valve was given by the leak detector.

The combination of fast valve action and baffles has shown to be effective in preventing not only the backflow of gas into the cyclotron vacuum system, but also any radioactive material carried along by the gas, from getting past the valve.

Electronic measurements of the time required for the valve to close indicated that (l) the time for the magnetic field to release the movable valve gate increases with increasing current applied to the magnet; (2) the time for the springs to close the valve decreases with increasing spring compression, and (3) these effects tend to cancel one another, such that the total time required for the field to collapse and the valve to close is approximately constant.

For the springs used in evaluation of the system (0.28 cm.- thick wire, compressed from 2.1 coils/cm. to 2.9 coils/cm. by raising the drive plate 69 (FIG. 2) a height of 3.5 cm.) the time required for the valve to close after its release by the electromagnet was determined to be ==8 msec. If it is assumed that the laws of simple harmonic motion may be applied to the spring and valve, the calculated maximum velocity obtained by the valve gate just before it slams shut is found to be =680 cm./sec. The total operating time was determined to be approximately 30 milliseconds, Moreover, the valve has been tested hundreds of times without apparent reduction in its speed or reliability.

The use of the baffles 29 is most significant. A valve system that inherently is not fast enough to stop a helium shock wave works quite well because of the addition of baffles to retard the shock wave. This result can be put to general use at accelerators or other vacuum operated systems, not only to reduce the chances of radioactive contamination but also to avoid shutdowns caused by accidental loss of vacuum in one line of a system. Therefore, it will be seen that a valve system for the protection of a vacuum system has been provided which has an apparent closing time somewhat longer than those of the less reliable prior art valves, but has been proved to be highly efiective and reliable for applications such as the protection of a cyclotron vacuum system following a target rupture in one of the beam tubes.

- What is claimed is:

1. In combination with a cyclotron vacuum system including at least one beam tube connected in open communication with said cyclotron vacuum system through which an accelerated particle beam from said cyclotron travels, a target chamber having a pressurized gas contained therein connected to the downstream end of said beam tube and a metal foil target window sealably disposed over an opening of said target chamber in beam receiving alignment with said beam tube, normally providing a gas-tight tight seal between said beam tube and said pressurized target chamber, a fast-closing valve system for protecting the cyclotron vacuum system from the backwash of gas from said target chamber in the event of a rupture in said target, comprising:

a valve housing positioned in said beam tube between said cyclotron and said target chamber, said housing having therein a valve chamber and aligned ports on opposite sides of an opening into said valve chamber through which said beam travels;

a valve gate slidably mounted in said valve chamber;

means for normally urging said valve gate into an extended position, closing said opening and interrupting said beam;

means including an electromagnet for holding said valve gate in a retracted position against the force of said urging means during normal operation, when said electromagnet is energized;

pressure sensing means disposed in said beam tube adjacent said target window for sensing a minute rise in pressure in said beam tube indicating a target rupture and deenergizing said electromagnet, thereby closing said valve gate; and

a plurality of baffles disposed in said line between said valve housing and said pressure sensitive means for attenuating a pressure shock wave generated by the rush of gas from a rupture in said target, thereby allowing time for said valve gate to close before said backwash of gas reaches said valve housing.

2. A fast-closing valve system as set forth in claim 1 wherein said pressure sensing means includes a spark gap device having spaced electrodes disposed in said beam tube closely adjacent to said target window well out of the path of said beam, a source of high voltage impressed across said electrodes, whereby a minute pressure rise in said line causes said spark gap to arc, shorting said electrodes, an adjustable current source connected to said electromagnet for supplying energiz ing current to said electromagnet, and switching means for sensing the arcing of said spark gap and shorting said electromagnet, thereby closing said valve gate.

3. A fast-closing valve system as set forth in claim 2 wherein said beam tube is circular in cross section and said plurality of bafiles includes a first plurality of annular rings positioned adjacent said target window in a predetermined spaced apart relationship and having aligned central openings disposed for free passage of said beam therethrough and a second plurality of annular rings positioned adjacent the downstream opening of said valve housing in the same said predetermined spaced apart relationship and having aligned central openings disposed for free passage of said beam therethrough.

4. A fast-closing valve system as set forth in claim 3 wherein said first and second pluralities of bafiles are equally spaced apart by distances of approximately 1 to 3 beam tube diameters. 

1. In combination with a cyclotron vacuum system including at least one beam tube connected in open communication with said cyclotron vacuum system through which an accelerated particle beam from said cyclotron travels, a target chamber having a pressurized gas contained therein connected to the downstream end of said beam tube and a metal foil target window sealably disposed over an opening of said target chamber in beam receiving alignment with said beam tube, normally providing a gas-tight tight seal between said beam tube and said pressurized target chamber, a fast-closing valve system for protecting the cyclotron vacuum system from the backwash of gas from said target chamber in the event of a rupture in said target, comprising: a valve housing positioned in said beam tube between said cyclotron and said target chamber, said housing having therein a valve chamber and aligned ports on opposite sides of an opening into said valve chamber through which said beam travels; a valve gate slidably mounted in said valve chamber; means for normally urging said valve gate into an extended position, closing said opening and interrupting said beam; means including an electromagnet for holding said valve gate in a retracted position against the force of said urging means during normal operation, when said electromagnet is energized; pressure sensing means disposed in said beam tube adjacent said target window for sensing a minute rise in pressure in said beam tube indicating a target rupture and deenergizing said electromagnet, thereby closing said valve gate; and a plurality of baffles disposed in said line between said valve housing and said pressure sensitive means for attenuating a pressure shock wave generated by the rush of gas from a rupture in said target, thereby allowing time for said valve gate to close before said backwash of gas reaches said valve housing.
 2. A fast-closing valve system as set forth in claim 1 wherein said pressure sensing means includes a spark gap device having spaced electrodes disposed in said beam tube closely adjacent to said target window well out of the path of said beam, a source of high voltage impressed across said electrodes, whereby a minute pressure rise in said line causes said spark gap to arc, shorting said electrodes, an adjustable current source connected to said electromagnet for supplying energizing current to said electromagnet, and switching means for sensing the arcing of said spark gap and shorting said electromagnet, thereby closing said valve gate.
 2. A fast-closing valve system as set forth in claim 1 wherein said pressure sensing means includes a spark gap device having spaced electrodes disposed in said beam tube closely adjacent to said target window well out of the path of said beam, a source of high voltage impressed across said electrodes, whereby a minute pressure rise in said line causes said spark gap to arc, shorting said electrodes, an adjustable current source connected to said electromagnet for supplying energizing current to said electromagnet, and switching means for sensing the arcing of said spark gap and shorting said electromagnet, thereby closing said valve gate.
 3. A fast-closing valve system as set forth in claim 2 wherein said beam tube is circular in cross section and said plurality of baffles includes a first plurality of annular rings positioned adjacent said target window in a predetermined spaced apart relationship and having aligned central openings disposed for free passage of said beam therethrough and a second plurality of annular rings positioned adjacent the downstream opening of said valve housing in the same said predetermined spaced apart relationship and having aligned central openings disposed for free passage of said beam therethrough.
 4. A fast-closing valve system as set forth in claim 3 wherein said first and second pluralities of baffles are equally spaced apart by distances of approximately 1 to 3 beam tube diameters. 